1. Field of the Invention
The present invention generally relates to a time base correcting apparatus and, more particularly, to a time base correcting apparatus for time-base-correcting a reproduced video signal reproduced by a rotary magnetic head from oblique tracks on a magnetic tape which is transported at a desired running speed that may be different from a tape running speed used when recording a video signal on the magnetic tape.
2. Description of the Prior Art
A conventional time base correcting apparatus of, for example, a BETACAM type VTR (video tape recorder) includes a memory into which a reproduced video signal (a luminance signal or color difference signals) reproduced from slant tracks on a magnetic tape by a rotary magnetic head is written or from which the same is read out. The time base correcting apparatus is further provided with a write in-line address counter for generating a write in-line address signal and a write line address counter for generating a write line address signal supplied to the memory, and a write clock signal generating circuit for generating a write in-line address increment clock signal and a write line address increment clock signal which are each synchronized with a reproduced horizontal synchronizing signal separated from the reproduced video signal. These signals are supplied to the write in-line address counter and the write line address counter, respectively. The time base correcting apparatus is also provided with a read in-line address counter for generating a read in-line address signal and a read line address counter for generating a read line address signal supplied to the memory, and a read clock signal generating circuit for generating a read in-line address increment clock signal and a read line address increment clock signal which are each synchronized with a reference horizontal synchronizing signal and are supplied to the read in-line address counter and the read line address counter. Thus, a jitter component (time base error) of the reproduced video signal is corrected.
The write clock signal and the write line address signal include the jitter component of the reproduced video signal, whereas the read clock signal and the read line address signal are each synchronized with the reference horizontal synchronizing signal so that while the read line address signal is generated at a constant speed, the write line address signal varies in accordance with the reproducing speed of the VTR. As a result, in a variable speed reproduction mode of the VTR, that is, a mode wherein reproduction is performed at a tape speed different from that used when recording a video signal on a magnetic tape, the write and read line address signals for the memories will frequently coincide with each other or overlap, so that the reproduced video signal cannot be correctly written in and read-out from the memory.
Heretofore, coincidence or overlapping of the write and read address signals has been avoided by the method discussed below with reference to FIGS. 1 and 2.
FIG. 1 diagrammatically shows an address arrangement of the write and read line address signals which are shown in sectors of inner and outer rings. These signals are used in the FWD (forward direction) variable speed reproduction mode and are supplied to a memory in which a video signal of, 32 line periods, for example, can be stored. In the FWD variable speed reproduction mode, the cycle of the horizontal synchronizing signal in the reproduced video signal becomes longer than that of the reference horizontal synchronizing signal so that the cycle of the write line address signal becomes longer than that of the read line address signal. Thus, as shown in FIG. 1, in the addresses behind address 3, for example, the addresses of write and read line address signals become coincident with each other. Therefore, in such event, n (integer) lines previously written in the memory are read out again so that the addresses of the signals do not coincide with each other.
Similarly to FIG. 1, FIG. 2 shows an addresses arrangement of the write and read line address signals in the REV (reverse direction) variable reproduction mode.
In the REV variable speed reproduction mode, since the cycle or the horizontal synchronizing signal in the reproduced video signal becomes shorter than that of the reference horizontal synchronizing signal, the cycle of the write line address signal becomes shorter than that of the read line address. signal. Therefore, as shown in FIG. 2, in the addresses behind address 3, for example, the addresses of write and read line address signals become coincident with each other. Then, in such event, n (integer) lines written in the memory are read out before the present line, so that the addresses of the signals do not coincide with each other.
A time base correcting apparatus in which the write address does not coincide with the read address is disclosed in
Japanese Patent Laid-Open Gazette No. 52-10022.
Because a conventional time base correcting apparatus is configured to reproduce a video signal which is, for example, sixteen horizontal periods ahead of the reference horizontal synchronizing signal, in the normal reproduction mode at a normal speed and in the FWD or REV variable speed reproduction mode at a speed slightly higher than the normal speed, addresses of write and reac address signals do not substantially coincide. By contrast, if the FWD and REV variable reproduction speeds become considerably higher than the normal speed, the addresses of the write and read address signals will eventually coincide. Further, since the reproduced video signal is sixteen horizontal periods ahead of the reference horizontal synchronizing signal, in the variable speed reproduction mode the addresses of the address signals do not coincide with each other at the upper area of a picture. However, in the lower area of a picture, the addresses of the address signals coincide with each other so that the above mentioned switching of read address lines is performed just before the reproduction of the lower area of the picture starts.
Accordingly, in the conventional time base correcting apparatus, a video signal is written in and read out from the memory correctly in a high variable speed reproduction mode, whereby the video signal can be reproduced as a video image on a monitor receiver.
However, the conventional time base correcting apparatus in which the video signal in the variable playback mode can be reproduced as a video image encounters the problems of so-called "picture contraction" and "picture expansion".
A fundamental principle of picture contraction and picture expansion will now be explained with reference to FIG. 3, which is a diagrammatic view of a section of a magnetic tape. In FIG. 3, reference numeral 101 designates a magnetic tape, and reference letters RT designate record tracks in which a video signal is recorded on the magnetic tape by a Betacam type video tape recorder (VTR). An arrow A represents a tape running direction in the forward reproduction mode and an arrow B represents the direction of reproducing head movement. Further, reference numeral HT1 represents a locus of the head in the normal reproduction mode, HT2 represents a locus of the head in the fast-forward reproduction mode and HT3 represents a locus of the head in the reverse reproduction mode. A cross mark X represents an alignment difference between the adjacent record tracks RT in the longitudinal direction of the track.
Incidentally, the rotational speed of the rotary head is made constant so that, in either the normal reproduction mode, the fast-forward reproduction mode or the reverse reproduction mode, the time required by the rotary head to move from the starting portion to the ending portion of each of the head loci HT1, HT2 and HT3 is equal. As shown in FIG. 3, since the track alignment difference X exists between the adjacent record tracks RT, the information amount of the head locus HT2 becomes smaller than the information amount involved in the head locus HT1 in the normal playback mode, and the information amount of the head locus HT3 becomes larger than the information amount of the head locus HT1 in the normal reproduction mode. As a result, as earlier noted, the cycle of the horizontal synchronizing signal of the reproduced video signal in the forward reproduction mode becomes longer than that of the reference synchronizing signal, whereas the cycle of the horizontal synchronizing signal of the reproduced video signal in the reverse reproduction mode becomes shorter than that of the reference synchronizing signal. If such a reproduced video signal is written in a memory in line units in accordance with the write line address signal and is read out from the memory in lines units in accordance with the read line address signal, the number of lines read-out during one cycle period (one field period) of the reference vertical synchronizing signal is constant. Therefore, with respect to the reproduced video signal obtained from the head moving along the locus HT2 in the fast-forward reproduction mode, information from the starting portion to the ending portion of the head locus HT2 is read out. However, with respect to the reproduced video signal obtained when moving along the head locus HT3 in the reverse reproduction mode, only the lines of one field period traversed by the starting end portion of the head locus are read out, and the remaining lines of that one field period will not be read. Consequently, as shown on FIG. 4, a circle in the normal reproduction mode is reproduced in the fast-forward reproduction mode as an ellipse elongated in the horizontal direction, that is, so-called "picture contraction" occurs. In the reverse reproduction mode, a circle in the normal reproduction mode is reproduced as an ellipse elongated in the vertical direction which has its lower portion "dropped out", that is, so-called "picture expansion" occurs, as shown in FIG. 5. The degree of "picture contraction" or "picture expansion" depends on the tape speed in the fast-forward or reverse reproduction mode.
Furthermore, the use of a conventional time base correcting apparatus for avoiding overlapping or coincidence of reading and writing addresses in the lower portion of the picture results in picture distortion, in addition to the problem of "picture expansion".